40mm/56 Gun
Page last updated: 24 September, 2016


40 mm/56 Mark 1, Mark 2 and M1

The Bofors 40 mm/56 was probably the best anti-aircraft weapon of World War II, and was used on almost every major US warship.

The Bofors Company was partly owned by German interests until 1930. The first prototype was fired on 17 Oct 30. It was not until 10 Nov 31 that automatic salvos were successfully fired.

The US Navy's BuOrd purchased an air-cooled twin version from Bofors in early 1940, and formally obtained Swedish licenses in June 1941. However, US manufacturers made radical changes to the Swedish design and as a result the guns and mountings produced in the USA bore little resemblance the Swedish original.

The 40 mm fuze designed and produced in Britain was adopted as an interim measure by the USA. This fuze was almost immediately replaced by one designed by R.L. Graumann of the Naval Ordnance Laboratory.  Simple in design and "ideally suited to mass production" the new fuze, designated Mark 27, was found to be 99.9 percent efficient in ballistic acceptance tests.

The first US Navy pilot twin was completed in Jan 42 and the first quad in Apr 42.  The first shipboard quad was installed on USS Wyoming (AG-17) on 22 Jun 42, and the first twin installation on the destroyer USS Coghlan (DD-606) on 01 Jul 42.  USA production of these weapons saw a total of about 39,200 built by the end of the war.

Late in World War II, the Navy began replacing 20 mm Oerlikons with 40 mm Bofors, as the 20 mm was found to be ineffective against Japanese Kamikazes.  (However, the Bofors was later determined to be inadequate against suicide attacks prompting Bofors to be replaced in the late 40's and 50's with the rapid fire 3"/50.

The development of the Mark 51 director system provided greatly improved accuracy.  (Half of all Japanese aircraft shot down between 01 Oct 44 and 01 Feb 45 were credited to the Bofors / Mark 51 combination.)

The Navy's Mark 1 and Mark 2 Bofors guns were both water-cooled and were used for all twin and quad mountings. The Mark 1 was a left-hand fed weapon, the Mark 2 was a right-hand fed weapon. These weapons could be fired in single-shot or automatic mode.

The M1 air-cooled version was originally produced for the US Army. All Navy single mount installations used a modified version of the M1.

The following is extracted from:


C. 40-mm Guns and Mounts

9C1. General

The 40-mm gun is a recoil-operated, heavy machine gun designed primarily for AA fire. Its distinctive features include (1) a vertical sliding-wedge breech mechanism, (2) a hand-fed automatic loader, (3) a spring-operated rammer, and (4) a trigger mechanism that controls the rammer operation only.

Once put in operation and the ramming cycle started, this gun loads and fires without further attention. It can be operated in either fully automatic or single fire. The maximum cyclic rate of automatic fire is about 160 rounds per minute.

For naval use, these guns are usually water-cooled and assembled in pairs, 1 pair making up a twin mount, 2 pairs a quad mount. The individual gun mechanisms are alike except for the changes necessary to make them right and left guns. Both twin and quad mounts are used on destroyer escorts, destroyers, and many classes of larger naval ships. Air-cooled single guns are used on some small craft.

The conventional mounts for twins and quads have power-operated elevating and training gear to position the guns as a unit. Power drives may be controlled at the mount by the pointer and trainer, or from the director through an electrical control system. The mounts can also be trained and the guns elevated manually by handwheels.

Ammunition is of the fixed type, loaded into clips containing four rounds each. The usual AA projectile is provided with a tracer and a nose-type impact fuze armed by the rotation of the projectile in flight. A complete round weighs about 5 pounds, the projectile about 2. The tracer is effective for from 7 1/2 to 8 seconds, after which it destroys the projectile by detonating the burster charge. Non-self-destructive AA and AP projectiles are also available. The service charge produces an initial velocity of about 2,800 feet per second. The maximum horizontal range obtained before self-destructive action, is about 5,000 yards; without self-destructive action, the range is increased to about 11,000 yards...........9C1. con't. (See Reference link below.)

9C2. Mount (See Reference link below.)

9C3. Personnel (See Reference link below.)



For interested non-Gunnersmates, CLICK HERE to go to a short list of terms and definitions used in Naval Gun discussions and literature.

(Provided by Gene L. Slover)

NavWeaps.com Naval Gun Page

Click HERE to link to a copy of the 40 MM Bofors Antiaircraft Gun manual OP 820 (1943)

Click HERE to link to a copy of the 40 MM Mount Mk 4, OP 1329 manual (1947)

Click HERE to link to a copy of the 40 MM Antiaircraft Gun manual TM 9-1252 (1951)

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